Acoustic analysis of gas mixtures

ABSTRACT

The present invention provides an acoustic cell for determining the composition of gas mixtures. The acoustic gas composition analysis cell has transducers acoustically isolated from the cell body through the use of an acoustic isolation material positioned at least between the transducers and the transducer housings to produce a signal-to-noise ratio of at least 4 to 1. Additionally, the transducers employed in the acoustic cell operate in the kilohertz range, reducing attenuation in the gas mixture being analyzed. The cell body employs vacuum seals which permit the use of the cell in line with vacuum equipment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of acoustic gas monitoring and, moreparticularly, to the in-line monitoring and control of the compositionof gas mixtures.

2. Description of the Related Art

In many manufacturing operations, accurate information concerning areaction gas composition is necessary to control a particular process.For example, chemical vapor deposition (CVD) processes require precisegas mixtures to reliably create materials of a specific composition.Formation of semiconductor materials and optical fiber preforms ofteninvolves incorporation of dopant materials in very small concentrations.The dopant material is supplied by a dopant precursor gas which is mixedwith other deposition gases in a reaction chamber. Because of the lowconcentration used in the vapor deposition process, the dopant gas isusually mixed with a carrier gas to ensure an even distribution ofdopant within the reaction chamber. The carrier gas must deliver aconsistent quantity of the dopant gas. In-line gas monitoring is oftenused to ensure this consistent deliver.

Acoustic monitoring of gases can employ ultrasound, i.e., sound waveshaving a frequency ranging from a few kHz to 10 MHz. Acoustic techniqueshave been extensively used for gas flow monitoring. More recently,efforts have turned to developing acoustic cells and processes which candetermine the concentration of a component of a binary gas mixture. Ingeneral, acoustic concentration analysis of a gas mixture is performedby measuring the speed with which sound waves propagate through a gasmixture. Because the speed at which the sound waves travel through a gasis related to molecular weight, the concentration of a component of agas mixture can be accurately detained.

For a single component system under ideal conditions, the velocity ofsound, V_(s) can be obtained from the following equation: ##EQU1## whereγ is the specific heat capacity ratio (C_(p) /C_(v)), R is the universalgas constant (8.3143 J/mol K), T is the absolute temperature in Kelvins,and M is the molecular weight of the gas in kg.

In the case of a binary gas mixture, a similar relationship exists, withγ and M replaced by γ and M. The acoustic velocity of a binary gasmixture is then represented by: ##EQU2## where γ is the average specificheat capacity ratio given by: ##EQU3## and M is the mean molecularweight of the binary gas mixture given by:

    M=(1-x)M.sub.1 +xM.sub.2

where x is the mole fraction of a second gas and M₁ and M₂ are therespective molecular weights of the first and second gases.

To solve for the concentration of a gas component, x, a quadraticequation is formulated from the above equations: ##EQU4## Where:A=constant=RT

a.tbd.γ₂ and b.tbd.γ₁,

c.tbd.M₂ and d.tbd.M₁.

x=concentration (mole fraction) of species corresponding to parameters aand c

(1-x) concentration (mole fraction) of species corresponding toparameters b and d

V_(s) is in units of meter/second.

This equation is solved for x using the quadratic formula. Thus, themeasurement of the velocity of sound through a binary gas mixture yieldsthe relative amounts of the two gas components.

The principle of acoustic gas analysis has been used in a gas monitoringcell shown in published UK Patent Application GB 2,215,049, thedisclosure of which is incorporated by reference herein. In thedisclosed cell, ultrasonic pulses are generated by an ultrasonictransducer. The transducer is composed of a piezoelectric material, suchas lead zirconate titanate, and is positioned opposite a secondtransducer. The transit time of sonic pulses between the transducers ismeasured and used to yield the sound velocity. From the velocity, thecomposition of the binary mixture is determined.

In the cell of the U.K. patent application, metal gaskets are employedfor gas sealing. Because these metal gaskets permit acoustic couplingthrough the body of the cell, each transducer is supported on an arrayof mounting pins to minimize acoustic coupling between the transducerand the cell body.

Although the cell of the U.K. application reduces acoustic coupling,there is still sufficient extraneous noise to interfere with theacoustic measurement process. The result is a loss of sensitivity of thecell. Additionally, the cell of the U.K. application operates usingultrasonic frequencies on the order of one megahertz. In general, assound frequency increases, the attenuation of sound waves alsoincreases. At frequencies in the megahertz range, attenuation of soundin the gas being analyzed is a problem, particularly when attempting tomeasure high sound absorptive gases, which absorb ultrasound in higherultrasonic frequency ranges. Because the cell of the U.K. applicationhas a short path length, higher frequencies are required to attain theresolution needed to detect the arriving pulse.

In U.S. Pat. No. 5,060,506, a method and apparatus are disclosed formonitoring the ratio of gases in a two-gas mixture using ultrasound. Thetransmitter used to generate the ultrasonic pulses is excited with asignal having a plurality of successive bursts, each of which includes apreselected number of excitation pulses at the resonant frequency. Theinitial pulse in each burst is separated from the final pulse in thepreceding burst by a quiescent time period of sufficient duration toassure dissipation of transients so that standing waves do not form.

There is a need in the art for improved acoustic cells and methods foranalyzing the composition of gas mixtures. More particularly, there is aneed in the art for acoustic cells which are compatible with vacuumenvironments without acoustic coupling of the transducer to the cellbody. Additionally, there is a need in the art for an acoustic gascomposition analysis cell which operates in a frequency range whichpermits measurement of a wide variety of gas mixtures.

Summary of the Invention

The present invention solves both the problems of unwanted acousticcoupling and vacuum compatibility by providing an acoustic gascomposition analysis cell having transducers acoustically isolated fromthe cell body to produce a signal-to-noise ratio of at least 4:1. Thetransducers employed in the acoustic cell operate in the kilohertzrange, reducing attenuation in the gas mixture being analyzed. The cellbody employs vacuum seals which permit the use of the cell in line withvacuum equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an acoustic gas analysis cell according to thepresent invention.

FIG. 2 is a side view in partial cross section of the acoustic gasanalysis cell of FIG. 1 illustrating the relationship of the transducerhousing to the gas conduit.

FIG. 3 is an enlarged side view in partial cross-section of a transducerand housing assembled in the acoustic gas analysis cell of the presentinvention, illustrating gas flow into the cell.

FIG. 4 is a schematic block diagram of the signal generating and dataprocessing circuit used with the acoustic gas analysis cell of FIG. 1.

DETAILED DESCRIPTION

Turning now to the drawings in detail in which like reference numeralsidentify like or similar elements in each of the several views, FIG. 1illustrates an exemplary acoustic gas composition analysis cellaccording to the present invention. The acoustic gas compositionanalysis cell 10 includes an electropolished stainless steel cylindricalconduit 20 for propagating sound waves through a gas mixture to beanalyzed. 316 stainless steel is an exemplary stainless steel forfabricating conduit 20. First and second transducer housings 40 arecoupled adjacent each end of conduit 20 by flange members 30.

As best seen in FIGS. 2 and 3, flange members 30 each include a hollowcylindrical portion 31 which coaxially surrounds conduit ends 22 todefine ring-like spaces 33. At one end, flange member cylindricalportion 31 is affixed to conduit 20 through weld 32. At its other end,flange member cylindrical portion 31 terminates in flange rim 34. Theflange rim 34 is provided with a plurality of through-holes 36 forreceiving threaded fasteners from a mating flange.

Mating flange 50 is an annular disk which engages transducer housing 40at housing lip 42 to position the transducer housing adjacent conduitend 22. Flange 50 includes through-holes 56 which align withthrough-holes 36 of flange rim 34 to receive threaded fasteners 52.Threaded fasteners 52 extend through both sets of through-holes 36 and56 and are engaged by receiving nuts 38.

Transducer housings 40 must be engaged with flange members 30 in anairtight manner to ensure compatibility of the gas analysis cell with avacuum system. For this reason, gaskets 60 are made from a metal such ascopper and are positioned between flanges 40 and 50. Gaskets 60 includeknife edges 64, best seen in FIG. 3, ensuring a leak-free seal whichresists degradation from noxious gases and can withstand vacuumevacuation in the ultrahigh vacuum range. When assembled, transducerhousings 40 position transducers 80 adjacent conduit ends 22 to definecylindrical gaps 39. Cylindrical gaps 39 are in fluid communication withthe ring-like space 33 between conduit 20 and flange member 30. The useof a separable transducer housing 40 facilitates replacement of thetransducer in the event of a transducer failure or the desire to use atransducer having a different resonant frequency.

To enable gas mixtures to flow through the acoustic cell, apertures 35are provided through the wall of flange member cylindrical portion 31.Gas ports 70 are welded to the flange member wall, enabling fluidcommunication with space 33 and cylindrical gap 39, best seen in FIG. 3.Each gas port 70 includes a mating member 72 such as a male VCR™ glandengaged within a threaded fastener 74 for facilitating connection to agas line. In use, one gas port serves as the gas inlet while the otherport serves as a gas outlet. As gas flows into a gas port, it passesinto ring-like space 33 and cylindrical gap 39 as illustrated by thearrows in FIG. 3. The gas then flows into the conduit 20 through openconduit ends 22. At the opposite end of the cell, the pathway isreversed, and gas exits through the remaining gas port. Advantageously,this gas pathway configuration promotes uniform flow distribution aroundthe transducer, minimizing the dead volume and improving the dynamicresponse of the device.

The transducer 80 is selected from piezoelectric materials, such aslead-zirconate-titanate crystals. The transducer thickness is chosen toachieve a desired resonant frequency, e.g., a resonant frequency in thekilohertz range, while acoustic isolation layer 82 and impedancematching thicknesses are comparable to the selected transducer layerthickness. Lead zirconate titanate transducers having resonantfrequencies in the range of 50 to 300 kilohertz and, more particularly,200-215 kilohertz are examples of transducers which can be used in theacoustic cells of the present invention. These frequency ranges are highenough to accurately measure acoustic signal time-of-flight in lowmolecular weight gases, but not so high as to incur absorption loss inthe gas being measured. Both transducers are acoustically matched, i.e.,they operate at the same frequency and impedance, thus increasing thesignal collected at the receiving transducer. The transducers operate atthe resonant frequency which produces the highest output per unit inputof electrical signal.

To accurately measure the transit time of sound waves within conduit 20,transducers 80 must be acoustically isolated from the transducer housingand other acoustic cell components. Acoustic isolation helps ensure thatsound is propagated through the gaseous mixture rather than through thevarious components of the acoustic cell housing. To this end, sleeves90, coaxially received within transducer housings 40, are provided tohouse transducers 80 and their affiliated components. Within each sleeve90, transducer 80 is coupled to a layer of acoustic isolation material82. Acoustic isolation material 82 can be an elastomeric material suchas a silicone elastomer. Material 82 attenuates radial sound from theresonating transducer and also assists in preventing extraneous noisefrom being transmitted through the housings and into the gas mixturebeing analyzed.

Because the acoustic impedance of a gas is substantially less than theacoustic impedance of the transducer, an impedance matching material 83is optionally positioned between transducers 80 and the gas mixture tobe analyzed. The term "impedance matching" is used to denote a materialhaving an acoustic impedance which is intermediate that of the gas andthe transducer. Impedance matching material 83 can be an elastomericmaterial such as a silicone, and can be formed separate from acousticisolation material 82 or can be integrally formed with acousticisolation material 82 as shown. Advantageously, use of an impedancematching material increases the acoustic power at the vibrating surfacewhich can be coupled into the gas to be analyzed by reducing reflectionof longitudinal sound waves. Increased power permits sound measurementswell above the noise level in the system, i.e., to produce a system witha high signal-to-noise ratio. A signal-to-noise ratio of 4:1 issufficiently high to obtain adequate time-of-flight measurement, withsignal-to-noise ratios of greater than 10:1 being exemplary.

For applications where exposure to the gas mixture would degradeimpedance matching material, a thin layer 84 of inert metal, such asgold, can be deposited over the impedance matching material. Layer 84can be deposited by vapor deposition or sputtering. The thickness oflayer 84 is selected to be effectively transparent to transmission ofsound waves by the transducer while protecting the underlying material.Thicknesses of 1-2 microns have been found to have thesecharacteristics.

To effectively absorb acoustic energy radiating from the back surface,space 85 and backing material 86 are positioned behind transducer 80.Spacing 85 is selected to be 1/4 wavelength of the operating frequencyof the transducer to improve acoustic attenuation. Materials such asfoams and other sponge-like materials are examples of backing material86. Optionally the backing material is supported by a sound reflectingwall to reflect sound into the backing material, useful for very shorttime-of-flight acoustic signals. Depending upon the desired acousticimpedance of the region behind transducer 80, the entire area may befilled with a backing material or an air gap. The backing material 86and space 85 also effectively attenuate extraneous noise transmittedthrough the transducer housing 40 and sleeve 90 by the remaining cellcomponents, improving the signal to noise ratio. Additionally, thebacking material minimizes "ringing" of the transducer, i.e., theundesirable radial vibrations due to even resonances.

Sleeve 90, coupled to acoustic isolation material 82 and transducer 80,is assembled with backing material 86 to transducer housing 40 throughthe use of a fixing agent 95. The fixing agent serves both to mount thecomponents in their proper position within transducer housing 40 andhermetically seal the unit for use with vacuum systems. Additionally,the fixing agent electrically isolates the transducer leads from thecell body. For this reason, the fixing agent can be an epoxy resinselected for compatibility with vacuum systems, such as TORR SEAL®,available from Varian Corporation, Lexington, Mass. The fixing agent 95assists in attenuating extraneous noise, further promoting acousticisolation of the transducers as well as performing its vacuum sealingfunction.

Transducer leads 87 and 88 pass through backing material 86 and fixingagent 95 to connect the transducers to a signal generating and dataprocessing circuit. Typically, leads 87 and 88 are molybdenum wiresformed sufficiently thin so that the effective impedance is much lessthan that of the transducer crystal to achieve a mismatch loss of energythat would otherwise travel through the leads. In general, the acousticcell of the present invention may be connected to a variety of signalgenerators/data processors. The basic characteristics of such systemsare that they stimulate a transmitting transducer to generate anacoustic signal and that they record an acoustic signal from a receivingtransducer, measuring the transit time of the signal between thetransducers. The system processes this information, yielding the soundvelocity and concentration of a gas component.

An example of a preferred signal generation/data processing system whichmay be employed with the acoustic cells of the invention is illustratedschematically in FIG. 4. The transducers 80 are connected to a signalgenerator 110, for transmission of ultrasound, and to a data acquisitiondevice 120 for receiving ultrasound. A switch 130 alternates theconnection of the transducers between signal generator 110 and dataacquisition device 120, permitting each transducer to be used as eithera transmitting or a receiving transducer.

Both the signal generator 110 and the data acquisition device 120 arecontrolled by microprocessor 150 and counter/timer 140. Themicroprocessor, in addition to controlling the circuit, processes thetransit times of the sound waves, using the transducer separationdistance to obtain the velocity. The microprocessor uses the velocity tosolve the quadratic equation for the concentration of a gas mixtureconstituent. The microprocessor may further be connected to a feedbackloop which controls the flow of the gas constituent in response to themeasured concentration.

An example of a signal generator/receiver system which may be employedwith the acoustic cells of the invention is Panametrics Inc. Model5055PR. In this device, the transmitting transducer is energized by atrigger-type, pulse-forming circuit that produces short pulses. Thereceived pulses are amplified, shaped, and then used to synchronize theoriginal pulse-forming circuit.

A further example of a system used to generate and receive pulses whichmay be used with the acoustic cells of the invention is Panametrics Inc.Model 6068. This system drives the transmitter with a binaryphase-encoded signal, typically 4 to 20 cycles in length. The receivedsignal is digitized using a "flash" analog to digital (A/D) converterand determines the transit time by correlating the encoded transmittedsignal with the digitized received signal. When using this system, thevelocity of sound in the medium being analyzed must first beapproximated before an accurate measurement can be made. This is due tothe fact that a window must be centered around the receiving signal toreduce the range of sound wave measurements so that pattern recognitioncan be effective.

Because the velocity of sound in a gas is also dependent upon the gastemperature, for greatest accuracy it is important to conduct the soundmeasurements at the same temperature. To this end, the acoustic cell andgas lines can be positioned within a standard exhausted gas cabinetwhich provides constancy in the temperature of the system to within±0.1° C. Alternatively, a thermocouple can be positioned within theacoustic cell and the gas temperature can be input to the microprocessoralong with the transit time for determining the gas composition.

In use, the acoustic gas analysis cell 10 is coupled to a gas flow linethrough gas ports 70. To calibrate the system, the distance between thetransducers is accurately determined using inert gases such as argon andhelium. The transmitting transducer launches a pulse of sound at theresonant frequency which passes through the gas contained within conduit20. For lead zirconate titanate (PZT) transducers, this resonantfrequency is approximately 100 kHz to 250 kHz depending upon thethickness and geometry of the PZT material.

The transmitted pulse is detected by the receiving transducer which isaxially aligned with the transmitting transducer. Because thetransducers operate at the same frequency and are acoustically matched,either transducer 80 in the acoustic cell of FIG. 1 can be selected asthe transmitting transducer, the remaining transducer being used as thereceiving transducer. To improve measurement accuracy, the transducerscan be operated in an alternating mode, i.e., the transducersalternately send and receive signals. Using this mode of operation, thetransit time of the ultrasonic pulses is measured both with and againstthe direction of gas flow. The velocity is the average of the measuredupstream and downstream velocities.

Using the velocity, the composition of the gas mixture is obtained. Theconcentration of a particular gas component is calculated by solving forx in the above quadratic equation. A list of coefficients used to solvethe quadratic equation for various binary gas mixtures is given in Table1 below:

    ______________________________________                                        List of Coefficients for Quadratic Formulation                                Binary Gas a      b      c × 10.sup.-3                                                                   d × 10.sup.-3                                                                  A                                     ______________________________________                                        1.)  AsH.sub.3 /H.sub.2                                                                      1.269  1.405                                                                              77.946  2.01594                                                                              2,478.909                           2.)  PH.sub.3 /H.sub.2                                                                       1.289  1.405                                                                              33.9978 2.01594                                                                              2,478.909                           3.)  Ar/He     1.669  1.630                                                                              39.948  4.0026 2,478.909                           4.)  NH.sub.3 /H.sub.2                                                                       1.304  1.405                                                                              17.031  2.01594                                                                              2,478.909                           5.)  SiH.sub.4 /H.sub.2                                                                      1.241  1.405                                                                              32.118  2.01594                                                                              2,478.909                           6.)  H.sub.2 Se/H.sub.2                                                                      1.314  1.405                                                                              80.976  2.01594                                                                              2,478.909                           7.)  HCl/H.sub.2                                                                             1.399  1.405                                                                              36.461  2.01594                                                                              2,478.909                           8.)  N.sub.2 /H.sub.2                                                                        1.407  1.405                                                                              14.0067 2.01594                                                                              2,478.909                           9.)  CH.sub.4 /H.sub.2                                                                       1.305  1.405                                                                              16.043  2.01594                                                                              2,478.909                           10.) NH.sub.3 /N.sub.2                                                                       1.307  1.407                                                                              17.031  14.0067                                                                              2,478.909                           11.) GeH.sub.4 /H.sub.2                                                                      1.227  1.405                                                                              76.63   2.01594                                                                              2,478.909                           12.) TMI/H.sub.2                                                                             1.10   1.403                                                                              159.925 2.01594                                                                              3,102.481                                (trimethyl                                                                    indium)                                                                  13.) TMG/H.sub.2                                                                             1.10   1.403                                                                              114.825 2.01594                                                                              3,102.481                                (trimethyl                                                                    gallium)                                                                 ______________________________________                                    

The acoustic cells and methods described herein find application invarious chemical vapor deposition processes. Deposition of glasses forfiber optic preform fabrication is an example of such a chemical vapordeposition process. Mixtures of silicon tetrachloride, SiCl₄, withoxygen and mixtures of germanium tetrachloride, GeCl₄, with oxygen areused to build up glass layers on the inside wall of a rotating silicatube to produce a graded index profile. For multimode fibers, thedeposited layers become the core and the silica tube becomes thecladding. Further process parameters can be found in MacChesney et al.,Proc. IEEE, Vol. 62 (1974), p. 1280, the disclosure of which isincorporated by reference herein. The acoustic cells are used to controlthe relative proportions of SiCl₄ and oxygen and GeCl₄ and oxygen.

The acoustic cells and methods of the present invention can be usedindividually for measurement of binary gas mixtures, or in series andparallel combinations for measurement of gas mixtures having more thantwo constituents. For example, an acoustic cell may be placed in a gasline to measure a first binary gas mixture. This binary mixture may thenbe further mixed with a third gas and a second acoustic cell may be usedto control the relative proportions of the binary gas mixture and thethird gas. The third gas may be a single gas or may itself be a binarygas mixture, the relative concentrations of its components having beencontrolled by an acoustic cell upstream of the later mixing point.

While the foregoing invention has been described with respect to thepreferred embodiments, it is understood that various changes andmodifications such as those suggested above, but not limited thereto,may be made without departing from the scope of the claims.

We claim:
 1. An acoustic cell for determining the composition of a gasmixture comprising:a hollow conduit for containing a gas mixture to beanalyzed, said conduit having first and second ends; a first transducerhousing sealingly engaged with said first end of the conduit, said firsttransducer housing supporting a first transducer, and having an acousticisolation material positioned at least partially between said firsttransducer and said first transducer housing to acoustically isolatesaid first transducer from said first transducer housing; a secondtransducer housing sealingly engaged with said second end of theconduit, said second transducer housing supporting a second transducerand having an acoustic isolation material positioned at least partiallybetween said second transducer and said second transducer housing toacoustically isolate said second transducer from said second transducerhousing, a first gas port coupled to said first end of the conduit; anda second gas port coupled to said second end of the conduit; whereinsaid acoustic isolation of said first transducer from said firsttransducer housing and said acoustic isolation of said second transducerfrom said second transducer housing produces a signal-to-noise ratio ofat least 4 to
 1. 2. An acoustic cell as recited in claim 1 wherein saidfirst and second transducers are acoustically matched.
 3. An acousticcell as recited in claim 1 wherein said first and second transducersresonate at a frequency between 50 and 300 kilohertz.
 4. An acousticcell as recited in claim 1 wherein said acoustic isolation material isan elastomer.
 5. An acoustic cell as recited in claim 4 wherein saidacoustic isolation material is a silicone elastomer.
 6. An acoustic cellas recited in claim 1 further comprising impedance matching materialpositioned between said first transducer and said first end of saidconduit and impedance matching material positioned between said secondtransducer and said second end of said conduit.
 7. An acoustic cell asrecited in claim 6 wherein said impedance matching material is anelastomer.
 8. An acoustic cell as recited in claim 7 wherein saidimpedance matching material is a silicone elastomer.